The present invention relates to digital communication where Orthogonal Frequency Division Multiplexing (OFDM) is employed, and more particularly to estimation of the Doppler spread of an OFDM channel.
In wireless communications, the channel is typically time-varying. This can be due to movement of the transmitter, movement of the receiver, and/or changes in the communications environment. For cellular systems (such as the Global System for Mobile communication (GSM) and the Wideband Code Division Multiple Access system (WCDMA)) and for broadcast systems (such as Digital Audio Broadcasting (DAB) and Digital Video Broadcasting (DVB)), the major cause for large Doppler spread is relatively high speed movement of the communications terminal. The standards for DAB and DVB call for the use of OFDM in communicating information from the transmitter to a receiver.
In an OFDM system, a channel comprises a number of sub-carriers (henceforth referred to simply as “carriers”) that are independently modulated, each by its own data. The modulation can be in accordance with a number of well-known techniques, such as Quadrature Amplitude Modulation (QAM) or n-ary Phase Shift Keying (n-PSK). The baseband signal in an OFDM system is then the sum of these modulated sub-carriers. The baseband signal is then used to modulate a main radio frequency (RF) signal. An important aspect of demodulating such a signal (thereby retrieving the underlying baseband signal) involves processing it by a Fast Fourier Transform (FFT). An advantage of communicating by means of OFDM is that it allows for communication over highly time-dispersive channels using reasonable complexity at the receiver side.
Whether a channel should be considered highly time-dispersive or not depends on the symbol rate that is used by the system. As a rule-of-thumb, a channel might be considered as non-dispersive if the root mean square (rms) delay spread of the channel is less than 10% of the symbol duration. Thus advantages of OFDM become more pronounced as the supported data rate is increased, which is exactly the case for most of the emerging systems.
The way to handle large delay spreads for a system based on OFDM is to make use of a guard interval (GI). The GI (also referred to in the literature as a “cyclic prefix”, or “CP”) is simply a copy of the last part of an OFDM symbol that is sent before the actual symbol. This is schematically illustrated in FIG. 1, which shows a number of symbols. An exemplary one of the symbols 101 includes a last portion 103 that is transmitted as part of a preceding guard interval 105 (time flows from left to right in the figure). Other guard intervals are similarly formed from end portions of their immediately succeeding symbols.
It is well-known that for a system based on OFDM the effect of the time-dispersive channel, known as inter-symbol interference (ISI), can be avoided provided that the length of the GI, TG, is at least as long as the (maximum) duration of the impulse response of the channel, henceforth denoted Tm. Because of the ability of an OFDM system to handle large delay spreads, it is very suitable for so-called Single Frequency Networks (SFN), which might be used for broadcasting. (In a single frequency network, geographically spaced transmitters operate on a same frequency. To reduce interference, they are time synchronized with one another.)
Suppose that the information carrying part of the OFDM begins at t=0, and that the length of the guard interval is TG. If the channel has a maximum delay spread, Tm, the requirement on the start of the FFT window is given by−TG+Tm<t≦0.  (1)
Thus, as long as Tm<TG it is possible to avoid ISI if t is chosen according to equation (1). However, if Tm>TG the issue is to choose t such that the effect of ISI is minimized. For systems designed for use in a SFN, the guard interval is typically so large that the first situation is the likelier one.
Now, as discussed above, ISI free reception is possible whenever Tm<TG. However, this requires identifying the exact start of the information carrying part of the signal. For this reason, OFDM receivers include arrangements for estimating the timing and frequency of the received signal.
To further improve performance, OFDM receivers typically include channel estimators, whose job is to dynamically determine the channel response. This information is then used to enable the receiver to process the received signal in a way that compensates for the time dispersion effects of the channel.
A conventional way of determining the channel response in an OFDM receiver is to dedicate certain ones of the carriers for use in conveying pilot signals. The pilot signals contain known information that permits the channel estimator to determine the channel response on that carrier frequency by comparing the actually received signal with the signal known to have been transmitted (i.e., one that the receiver would have expected to receive under ideal channel conditions). The carriers conveying the pilot signals are spaced apart in frequency by an amount that permits the channel response of carriers lying in-between the pilot carriers to be accurately estimated by interpolating the channel responses determined for the pilot carriers.
FIG. 2 is a block diagram of an exemplary OFDM receiver. An analog signal, r(t), generated by receiving and downconverting (either to intermediate frequency or baseband) a radiofrequency signal, is supplied to an analog-to-digital (A/D) converter 201. The digitized signal, r(k), is then supplied to a coarse timing and frequency estimation unit 203, which generates a coarse estimate of the timing and frequency offset of the received signal. (The frequency offset is the difference between the frequency of the transmitted signal and the frequency of the received signal.) This information is supplied to a frequency correction unit 205 as well as a GI removal unit 207. The GI removal unit 207 also receives the output of the frequency correction unit 205. Based on the best timing and frequency information available, the GI removal unit 207 removes the GI and supplies the information part of the received signal to an FFT unit 209, whose output is supplied to the remainder of the receiver, including a refined timing and frequency estimation unit 211, which is able to generate more accurate timing and frequency information from the FFT output signal. The more accurate frequency information is fed back to the frequency correction unit 205 to improve its performance. The more accurate timing information is similarly fed back to the GI removal unit 207 to improve its performance.
The output of the FFT unit 209 is also supplied to a channel estimator 213, which generates a complete estimate of the channel response by interpolation, as explained above.
How quickly the channel is changing is often measured by the so-called Doppler spread or the maximum Doppler frequency, fD. The Doppler frequency is defined as
                              f          D                =                  v          ⁢                                          ⁢                                    f              c                        c                                              (        2        )            where v is the speed of the receiver in m/s, fc is the carrier frequency in Hz, and c is the speed of light in a vacuum (i.e., approximately 3·108 m/s).
It is important at this point, in order to ensure clarity of discussion, to define a number of issues, each associated with the word “Doppler”, that arise in communications contexts. These are:    1. A pure Doppler shift. This is what one encounters when dealing with a one-tap channel, and the result is a pure frequency error. The Doppler shift frequency error cannot be distinguished from a frequency error that is caused by a transmitter and receiver not using exactly the same frequency. A pure Doppler shift is relatively easy to estimate, and completely trivial to remove. This is done by effectively multiplying the received signal by a complex signal with minus the estimated Doppler shift.    2. Doppler spread. Where a communications channel is characterized by multi-path propagation, different paths will arrive at different angles and by that have different Doppler shifts. The maximum Doppler shift is obtained when the angle of arrival is 0 and pi (but with a different sign for the two angles), and all Doppler frequencies in-between are possible. In contrast to the case of a pure Doppler shift, Doppler spread cannot be easily compensated by multiplying with a complex signal. In communication systems, Doppler spread is often treated as a frequency error that simply cannot be removed, and this is considered for instance when channel estimation is to be performed. The effect of Doppler spread is also very much like a non-compensated frequency error.    3. Inter-Carrier Interference (ICI) due to Doppler spread. In OFDM systems, a pure Doppler shift is usually no problem for the same reason that it is not a problem in single carrier systems—it can easily be removed. If not removed, then it is a problem for the same reason as for single carrier systems and in addition because it causes FFT leakage. In a similar fashion as for single carrier systems, the effect of Doppler spread cannot be easily counteracted by a complex multiplication. Instead, ICI cancellation is a rather complex operation that is done after the FFT in an OFDM receiver (removing a pure Doppler would have been done prior to the FFT) and requires the channel to be accurately estimated.
The focus of the discussion and description of embodiments that follows is that described in paragraph “2” above, namely, the determination of Doppler spread in an OFDM communications system. It is assumed that the frequency offset has been removed prior to the FFT.
As discussed above, a high degree of Doppler spread implies that the channel is changing quickly. This, in turn, means that reception of a signal becomes more difficult if specific knowledge of the channel (like phase and amplitude) is required for properly demodulating the signal. Since it is often possible to determine what the highest Doppler frequency is that will be experienced under typical operating conditions, it is possible to design a receiver based on this Doppler frequency. However, in case the actually experienced Doppler frequency is significantly smaller, designing for the worst case means that unnecessarily complex algorithms are used for channel estimation.
Knowledge of the Doppler frequency can also be used to determine how frequently some algorithms in a receiver have to be activated. For example, knowledge of the Doppler frequency can be used to determine, among other things,                for a path searcher in a CDMA receiver, how often it is necessary to search for new paths in the impulse response;        how often algorithms related to handover between cells in a cellular communications system should be initiated (the higher the Doppler frequency, the more often such algorithms should be run, since a high Doppler frequency indicates faster movement of the receiver); and        in a single frequency network, how often to scan for a better frequency that communications can be handed off to.Thus, although it is possible to design a receiver for the worst case Doppler spread, it is usually a very wasteful approach.        
In addition, in instances in which the channel estimation is based on Wiener filters to further improve the performance, as described in U.S. patent application Ser. No. 10/920,928 entitled “Channel estimation by adaptive estimation in time”, by L. Wilhelmsson et. al., the actual Doppler frequency is needed in order to calculate the Wiener filter. Also, even if the channel estimation is not based on a Wiener filter approach, but on, for example, using filters of different complexity depending on how difficult the channel estimation is, an estimate of the Doppler frequency is needed.
DAB and DVB are just two of a number of systems in which the rate of channel variation can be considerable. In particular, for the newly developed DVB standard for Hand-held devices (DVB-H), it can be expected that services are used when the user is completely standing still, implying a close to stationary channel, as well as when the user is traveling in a moving vehicle (e.g., a car), implying that a significant Doppler is experienced. DVB-H is based on OFDM, and one of the most computationally intensive blocks of the DVB-H receiver is that where channel estimation is performed. As described in the above-referenced U.S. patent application Ser. No. 10/920,928, knowledge of the Doppler spread can be used to find a suitable interpolation filter to be used for channel estimation.
The accuracy of the Doppler estimation will usually improve the longer Doppler measurements are made. However, allowing a longer time for measurement means not only that the actual estimation will take longer, but also that the receiver will be less responsive to fast variations in the actual Doppler spread. Thus, the time for performing Doppler estimation should be as short as possible, but still long enough to guarantee that the required accuracy is obtained.
Consequently, there is a need for being able to perform Doppler spread estimation in an efficient way.